In fiction (not to mention Real Life), it's not uncommon for friends to be as close as family. For the most part such relationships go unformalised. You might wish you could make a friend an official relative, but your options are limited. Adoption is difficult, especially when both people involved are adults. And marrying your friend probably isn't an option if you just don't feel that way about each other.
But there is another option that some characters take: marry one of your friend's relatives! Or fix it so that one of your relatives marries one of theirs. Then you'll become in-laws, which is almost as good as being blood relatives, and gives you an excuse to spend vacations and holidays together. Some common variations:
- Marry your friend's sibling, so that your friend becomes your sibling-in-law.
- Marry your friend's child, so that your friend becomes your parent-in-law.
- Have a child who marries your friend's child, so that you and your friend become... um... well, in-laws of some sort, anyway.
Often in these situations the romantic relationship comes off as secondary to the platonic one. Sometimes the platonic relationship may even be the primary motivator for the marriage, though it's hoped that most characters genuinely Marry for Love.
Especially in modern times, when marriage rates are on the decline, a long-term non-married relationship can qualify for this trope, provided it brings two friends closer together and doesn't create tension between them.
- In Code Geass, Lelouch considers this a favourable outcome with his best friend/rival Suzako and sister Nunally. They might have hooked up in the end, but Lelouch isn't around to be an inlaw.
Film - Live Action
- At the end of the original Star Wars trilogy, Luke Skywalker and Han Solo seem to be headed in this direction.
- At the end of the Harry Potter series, Harry becomes Ron's brother-in-law by marrying Ginny Weasley. Since Hermione marries Ron, he also effectively becomes Hermione's brother-in-law.
- In the appendices to The Lord of the Rings, Sam's daughter Goldilocks marries Pippin's son Faramir.
- In the Star Wars Expanded Universe, this trope becomes canon for Luke and Han Solo.
- No one comes out and says it, but at the end of the Belgariad Durnik, Garion's oldest friend, technically becomes his uncle by marrying Polgara.
- Invoked in Friends when Ross discovers that Chandler's now in a relationship with his sister Monica and initially sees it as a case of My Sister Is Off-Limits.
Ross: (angry) I can't believe it, my best friend and my sister!
Chandler explains that it's a serious relationship.
Ross: (overjoyed) I can't believe it, my best friend and my sister!
- Doctor Who, when the Doctor marries River, leading to a funny moment when Amy realizes that she's now the Doctor's mother-in-law.
- Subverted with Jim and Andy in According to Jim: They didn't start off as friends but become this after Jim marries Andy's sister. Jim is also really close to his mother in law to the point where Andy thinks she prefers him as a son.